The present invention relates to novel chemiluminescent acridinium compounds that have emission maxima close to or in the near infrared (NIR) region (>590 nm). The structural requirements for such long-wavelength emitting acridinium compounds are disclosed herein. These novel acridinium compounds when used in conjunction with short-wavelength emitting acridinium esters (with emission maxima below 450 nm) should be highly useful labels for the simultaneous detection of multiple target analytes in immunoassays or nucleic acid assays due to the extremely small or negligible spectral overlap between the different labels. A true and effective optical segregation of the two or more specific signals associated with different analytes can be easily achieved by the use of optical filters in conjunction with a simple algorithmic operation for minor correction of any low level of cross-talk. From a practical standpoint, minimal spectral overlap is thus the key factor that is necessary for the simultaneous and accurate measurement of different analytes. A further application of the compounds described here is in situations where there is optical interference from biological samples at short-wavelength (under 500 nm). Under these conditions, these novel acridinium compounds should be useful labels, permitting the optical separation of non-label related luminescence from the specific, chemiluminescent signal generated from the binding complex. Finally, long-wavelength acridinium compounds used in conjunction with detectors such as CCD cameras offer the potential for improved assay sensitivity since these detectors are remarkably efficient in reading long-wavelength signals.